The present invention relates to radio base stations and methods for controlling the radio base stations and more particular, to a radio base station which can suppress interference between radio base stations connected to a wired LAN and interference caused by external noise and can enhance the performance of a wireless system, because such radio base stations automatically optimumly set their own communicatable ranges and operating frequencies. The present invention also relates to a method for controlling the radio base stations.
At present, a wireless LAN is arranged generally based on a communication system using a direct spread (DS) system. The DS system is a technique for attaching a code monopolizing a specific frequency band to a signal, spreading and transmitting it. The number of radio terminals wirelessly connectable to a single radio base station is practically about 20. When it is desired to use about 20 or more of the radio terminals, it is required to increase the number of such radio base stations.
In the wireless LAN based on the DS system, the radio base station monopolizes and transmits the specific frequency band. Thus, when a plurality of radio base stations are located in an identical area, a technique for locating the radio base stations so as to avoid interference between the stations is required.
FIG. 16 is a diagram for explaining how to set the operating frequencies of radio base stations in a general radio LAN. In FIG. 16, reference numeral 150 denotes a wired LAN, reference symbols 151A and 151B denote radio base stations, symbols 152A and 152B denote radio terminals, and 153A and 153B denote communicatable areas.
FIG. 16 shows an example of a configuration of a system when having the two radio base stations 151A and 151B connected to a wired LAN 150. As shown in FIG. 16, it is assumed that the radio base stations 151A and 151B have the radio terminals 152A and 152B to be wirelessly connected to the respective radio base stations respectively. When the radio base stations have the respective communicatable areas 153A and 153B, the communicatable area of one of the radio base stations includes the communicatable area of the other. For this reason, there is a chance that radio wave interference may take place between the radio base stations. Thus, for the purpose of avoiding such mutual interference between the radio base stations, it becomes necessary to assign different communication frequencies to the radio base stations.
The technique shown in FIG. 16, for the purpose of avoiding the interference as in the aforementioned case, is to automatically set different operating frequencies for the respective radio base stations. For this reason, each radio base station is arranged so as to keep track of set and quality information of other radio base stations and the number of other radio base stations as viewed from the concerned radio base station, to give priorities to the automatic settings of frequencies, and to optimumly determine set frequencies. Such a technique is effective when a plurality of radio base stations are used in an identical area.
As an example of such radio LAN techniques, a technique disclosed, e.g., in JP-A-2002-217918 is known.
In a radio LAN based on the above technique, in order to effectively use an available or usable frequency band, the frequency of each radio base station is set to a predetermined value, which is called channel. For example, in the case of a 2.4 GHz band radio LAN, a range of 97 MHz between 2.4 GHz and 2.497 GHz is divided into 14 channels as usable frequency ranges. In the case of a 5 GHz band radio LAN, a range of 100 MHz between 5.15 GHz and 5.25 GHz is divided into 4 channels as usable frequency ranges.
In the case of the 2.4 GHz band radio LAN, it has 14 channels as mentioned above. When a plurality of radio base stations are present in a communicatable area, however, it is necessary to set the frequencies or channels of the base stations with a distance spaced by an about 2 channels.